Here is list of shops that sell items for the popen, AKA the Open Pony body on Second Life. This list includes accessories, textures, items, mods, animations, and furniture intended for the OP. Currently these are listed in no particular order. I am just pulling stores by the order which I found them while making this list. Here are some disclaimers:
I am including every shop that has any popen items, even if it's a SINGULAR ITEM. Some of these shops will include items that are for other bodies, cater to unusual fetishes, nohooves items, or are just overall cringe. I will try to leave disclaimers for these shops as I edit this list, but for now I am just going to dump links. 
If you can't find the popen friendly items in a shop, use the search function. Searching "pony" or "op" will usually get you the results you want. 
There will be stores which sell items that might not be show accurate or even ponylike in nature. This list is intended to be a comprehensive, so I won't be leaving anything out. Don't shoot the messenger for sharing a few bad stores!
This list is a work in progress. Many stores are missing and this list will be updated as needed and as I'm able to.
Eventually I'll sort out the stores in categories with proper tags and disclaimers. I may also include a list of tangentially related stores such as stores that sell MLP gestures, sounds, items for other pony bodies, or unrigged hair that works well as a mane. For now, I am listing for popen only. 
If you know of a store that isn't listed here, post it in the thread for me to add.
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Hmart: https://marketplace.secondlife.com/stores/161770
LilPest: https://marketplace.secondlife.com/stores/248646
tree sand: https://marketplace.secondlife.com/stores/106758
[soff]: https://marketplace.secondlife.com/stores/234578
Mourningstar: https://marketplace.secondlife.com/stores/231787
Iferdim ~ Digital Accessories: https://marketplace.secondlife.com/stores/260125
Backspace Industries: https://marketplace.secondlife.com/stores/252513
"Store": https://marketplace.secondlife.com/stores/248756
Orchidpony: https://marketplace.secondlife.com/stores/251177
Dox Box: https://marketplace.secondlife.com/stores/146947
biepot: https://marketplace.secondlife.com/stores/257698
[ Zero Nine ]: https://marketplace.secondlife.com/stores/220138
Middry: https://marketplace.secondlife.com/stores/168170
~ STORE DESU ~: https://marketplace.secondlife.com/stores/205650
The Preening Pegasus: https://marketplace.secondlife.com/stores/248470
[MNM] Melody Nova's Market: https://marketplace.secondlife.com/stores/209705
Horse Things: https://marketplace.secondlife.com/stores/249223
OCEANIC: https://marketplace.secondlife.com/stores/250154
Cheetoidea: https://marketplace.secondlife.com/stores/249150
~ Café Muse ~: https://marketplace.secondlife.com/stores/188095
Pony Skins: https://marketplace.secondlife.com/stores/248719
Wizards R Us: https://marketplace.secondlife.com/stores/243351
Naturalistic: https://marketplace.secondlife.com/stores/248889
bogeeee store: https://marketplace.secondlife.com/stores/250839
Peach Bunny: https://marketplace.secondlife.com/stores/245449
The Mailbox: https://marketplace.secondlife.com/stores/242127
Safeguard Shipping Co: https://marketplace.secondlife.com/stores/182288
::DarkChamber:: : https://marketplace.secondlife.com/stores/216886
PastelCloud: https://marketplace.secondlife.com/stores/167554
Demo26s: https://marketplace.secondlife.com/stores/261087
Royal.: https://marketplace.secondlife.com/stores/261084
The Conjuring Wolf: https://marketplace.secondlife.com/stores/198120
Gotten Goat Shoppe: https://marketplace.secondlife.com/stores/259414
CADNOMORI: https://marketplace.secondlife.com/stores/237530
The Wooly Pig: https://marketplace.secondlife.com/stores/86456
{:: The Moonstone Studio ::}: https://marketplace.secondlife.com/stores/222770
WOLFKIT: https://marketplace.secondlife.com/stores/235489
LuminousDazzle: https://marketplace.secondlife.com/stores/254184
Mooshie: https://marketplace.secondlife.com/stores/186020
Haus Dripp: https://marketplace.secondlife.com/stores/249548
Worst store in sl: https://marketplace.secondlife.com/stores/248557
[Eitherworld]: https://marketplace.secondlife.com/stores/161639
Pink Horse: https://marketplace.secondlife.com/stores/220026
Allyster Black: https://marketplace.secondlife.com/stores/83649
Moss Hollow: https://marketplace.secondlife.com/stores/257852
Malpractice: https://marketplace.secondlife.com/stores/255634
Marshmallow Pudding Co.: https://marketplace.secondlife.com/stores/253823
QuietRespite: https://marketplace.secondlife.com/stores/222920
♡ Minimuzzle ♡: https://marketplace.secondlife.com/stores/222005
Flufftop: https://marketplace.secondlife.com/stores/225279
dreamyribs: https://marketplace.secondlife.com/stores/170337
InuInul: https://marketplace.secondlife.com/stores/242072
Caboodle: https://marketplace.secondlife.com/stores/163749
Black & White: https://marketplace.secondlife.com/stores/203088
BigBone: https://marketplace.secondlife.com/stores/233770
Coffee and Co.: https://marketplace.secondlife.com/stores/243390
CONTACT.: https://marketplace.secondlife.com/stores/136195
The Harvest's Bounty: https://marketplace.secondlife.com/stores/233700
♡ Multiverse - Dev lab: https://marketplace.secondlife.com/stores/220769
ziTems: https://marketplace.secondlife.com/stores/262088
ThatPawsomeFurry: https://marketplace.secondlife.com/stores/116797
JOINT EFFORT: https://marketplace.secondlife.com/stores/183689
Hind's Horse Hole: https://marketplace.secondlife.com/stores/249859
Mimi's Fashion: https://marketplace.secondlife.com/stores/134132
The Witches Bruume: https://marketplace.secondlife.com/stores/49557
BonBon Designs: https://marketplace.secondlife.com/stores/261657
=Dreamstuck=: https://marketplace.secondlife.com/stores/132119
Far-off pawductions: https://marketplace.secondlife.com/stores/260773
Touryuumon :Mahō Sukurōru: https://marketplace.secondlife.com/stores/10784
Black Vinegar: https://marketplace.secondlife.com/stores/152168
CLOVER.: https://marketplace.secondlife.com/stores/247658
Fox in the Box Designs: https://marketplace.secondlife.com/stores/90013
Munmochi: https://marketplace.secondlife.com/stores/229388
The Gren Stor: https://marketplace.secondlife.com/stores/251174
DISCO EMPORIUM: https://marketplace.secondlife.com/stores/255550
.eligius.: https://marketplace.secondlife.com/stores/257125
Edgy*Kittiee™: https://marketplace.secondlife.com/stores/201172
Baiku's Emporium Of Random Things: https://marketplace.secondlife.com/stores/176815
Popen-Source: https://marketplace.secondlife.com/stores/254240
{Sweet Buns}: https://marketplace.secondlife.com/stores/171504
Buzz & Flutter: https://marketplace.secondlife.com/stores/250414
The Rusty Lion: https://marketplace.secondlife.com/stores/112183
DiamondBrony: https://marketplace.secondlife.com/stores/257314
The Cauldron: https://marketplace.secondlife.com/stores/93464
Pegasus Industries: https://marketplace.secondlife.com/stores/186570
Cloven Spice: https://marketplace.secondlife.com/stores/261103
The Swannery: https://marketplace.secondlife.com/stores/230002
Casuarina Collection: https://marketplace.secondlife.com/stores/254193
Obvious Shenanigans: https://marketplace.secondlife.com/stores/189176
+SST+ Shadow's Studios: https://marketplace.secondlife.com/stores/135970
..:::Wylde♡Candy:::.. : https://marketplace.secondlife.com/stores/69252
Silver & Wolf: https://marketplace.secondlife.com/stores/18511
Illadari's odds & ins: https://marketplace.secondlife.com/stores/261457
R. H. Creation´s: https://marketplace.secondlife.com/stores/171439
Mousey's shop: https://marketplace.secondlife.com/stores/262331
Art "N" Prints: https://marketplace.secondlife.com/stores/229066
KyaaPon.: https://marketplace.secondlife.com/stores/248936
[Butter Bean]: https://marketplace.secondlife.com/stores/86075
Sin.: https://marketplace.secondlife.com/stores/176991
Trash Can Elite: https://marketplace.secondlife.com/stores/172426
OKAPPY!: https://marketplace.secondlife.com/stores/245320
NORDING Delivery: https://marketplace.secondlife.com/stores/207290
~Shapes 'n' Sizes~: https://marketplace.secondlife.com/stores/175002
BlueMoon Designs: https://marketplace.secondlife.com/stores/158207
MoonMush: https://marketplace.secondlife.com/stores/250283
Furry Andy: https://marketplace.secondlife.com/stores/116095
Neomi's Bag of Stuff and Things!: https://marketplace.secondlife.com/stores/243262
Nova Xanthe: https://marketplace.secondlife.com/stores/237920
Dramatis: https://marketplace.secondlife.com/stores/130000
[ The Cosmic Rabbit ]: https://marketplace.secondlife.com/stores/76052
тooтs: https://marketplace.secondlife.com/stores/167499
Lavender Geode: https://marketplace.secondlife.com/stores/245952
[LSLS]: https://marketplace.secondlife.com/stores/242593
Silph Co: https://marketplace.secondlife.com/stores/244293
FOUND FOOTAGE: https://marketplace.secondlife.com/stores/99776
[Debutante]: https://marketplace.secondlife.com/stores/243298
+CAF+:https://marketplace.secondlife.com/stores/24488
Moon Pop: https://marketplace.secondlife.com/stores/195548
fhaxaris's emporium: https://marketplace.secondlife.com/stores/248809
Trendy Trotters: https://marketplace.secondlife.com/stores/229308
Mama's Hoard: https://marketplace.secondlife.com/stores/248230
*Star Kluster*: https://marketplace.secondlife.com/stores/206094
+Solemn Swears+: https://marketplace.secondlife.com/stores/213501
Ankoku Stourmead: https://marketplace.secondlife.com/stores/149745
✧ Crazy world ✧: https://marketplace.secondlife.com/stores/197629
ANAGUMA CORPORATION: https://marketplace.secondlife.com/stores/2100
✧AR✧: https://marketplace.secondlife.com/stores/171263
VinylSS: https://marketplace.secondlife.com/stores/249697
{Cᴀᴄᴛᴜs Pᴀʀᴀᴅᴇ}: https://marketplace.secondlife.com/stores/203750
Fluff & Teeth: https://marketplace.secondlife.com/stores/163077
Milky Fox: https://marketplace.secondlife.com/stores/161721
_.ArtGuts._: https://marketplace.secondlife.com/stores/169951
Berry'd Treasure: https://marketplace.secondlife.com/stores/249043
//ANGELHIVE//: https://marketplace.secondlife.com/stores/198151
Horkus Porkus: https://marketplace.secondlife.com/stores/117876
*Momo*: https://marketplace.secondlife.com/stores/258706
Candy Critters: https://marketplace.secondlife.com/stores/166168
• MEXICAN DEMON •: https://marketplace.secondlife.com/stores/243318
Sus!: https://marketplace.secondlife.com/stores/248961
CookieCakeArts: https://marketplace.secondlife.com/stores/253297